Gadolinium ions inhibit exocytotic vasopressin release from the rat neurohypophysis.

Single rat neurointermediate lobes (n.i.l.s) were fixed by their stalks to a platinum wire clip electrode and incubated in oxygenated Krebs-HEPES medium. Vasopressin release int the medium was determined by radioimmunoassay. Vasopressin secretion was increased by different stimuli and the effects of gadolinium (Gd3+) were tested. Electrical stimulation (15 Hz, three times 1 min with 1 min intervals) increased vasopressin release in a calcium-dependent manner. Gd3+ (10 microM to 3 mM) inhibited the evoked release of vasopressin in a concentration-dependent fashion; at 3 mM the inhibition was 98%. The inhibitory effect of Gd3+ up to 300 microM was antagonized by increasing the calcium concentration in the medium up to 6 mM. The effects of 1 and 3 mM-Gd3+ were unaffected by increasing the calcium concentration. Exposure of n.i.l.s to depolarizing concentrations of potassium (high K+, 60 mM, 30 min) increased the vasopressin release more than 33-fold. The elevated vasopressin release remained constant during six consecutive 5 min periods. In the initial 5 min period 300 microM-Gd3+ reduced the evoked vasopressin release by 80% but during the last 5 min period only by 30%. At 3 mM-Gd3+ vasopressin release was completely blocked during the whole time of incubation with high K+. Vasopressin release induced by exposure of n.i.l.s to cold (4 degrees C, 20 min) was completely inhibited by 3 mM-Gd3+, but reduced by only 25% in the presence of 300 microM-Gd3+. Vasopressin release induced by incubation of n.i.l.s with the ionophore X-537A (lasalocid) (10 microM, 30 min) was reduced by 90% in the presence of 300 microM-Gd3+ and completely prevented by 3 mM-Gd3+. 300 microM-Gd3+, added to the incubation medium, had no significant effect on the vasopressin release from crude synaptosomal preparations evoked by high K+. However, when 300 microM-Gd3+ was already present during the tissue homogenization, the evoked vasopressin release from the synaptosomes was completely blocked. It is concluded that Gd3+ inhibits exocytotic vasopressin release at two different sites. First, Gd3+ may block voltage-regulated calcium channels. Secondly, Gd3+ may inhibit the exocytotic release mechanism by an intracellular site of action. It is speculated that contractile proteins may be the intracellular target for Gd3+.